Generally, the imaging apparatuses using X rays, ultrasonic waves and MRI (magnetic resonance imaging) find wide application in the medical field. Also, in the medical field, the vigorous research effort is promoted on the optical imaging apparatus in which the light radiated on a living organism from a light source of a laser, etc. is propagated through a subject such as an organism, and by detecting this propagated light, the information inside the organism is obtained. An example of this optical imaging technique is photoacoustic tomography (PAT).
The PAT is a technique for visualizing the information related to the optical characteristic values inside a subject (See PTL 1: U.S. Pat. No. 5,713,356). Specifically, upon radiation of the pulse light on a subject from a light source, a photoacoustic signal (also called an acoustic wave, or typically, an ultrasonic wave) is generated, propagated and diffused from the tissue of the organism that has absorbed the optical energy. The temporal change in the acoustic wave is detected at a plurality of points surrounding the organism, and the signal thus obtained is analyzed. In this way, the optical characteristic values such as the initial pressure generation distribution caused by the light radiation in the subject and the optical energy absorption density distribution can be obtained.
The absorption spectra of oxidized hemoglobin and reduced hemoglobin in the blood, for example, show that the absorption amount changes at the wavelength of about 800 nm. By radiating the light in the wavelength of about 800 nm for measurement, therefore, the oxygen saturation degree of the blood can be determined. As a result, the point of a malignant tumor accompanied by the proliferation of new blood vessels, for example, can be specified. In the case where the distribution of the optical characteristic value of the tissue of an organism is determined in the comparative neighborhood of the surface of the organism, however, a wider wavelength range of, say, 400 nm to 1600 nm can be also used. In the case where the subject is not an organism, on the other hand, the wavelength range is not limited to the figures described above in an actual application.
It is generally known that an acoustic wave of a stronger sound pressure can be obtained by a shorter pulse width of the radiated light for an improved SN ratio. The pulse laser is often used, therefore, in the research on the photoacoustics, or especially, an application to an organism large in light attenuation. More specifically, the Q-switch YAG laser (wavelength of 1064 nm) is widely used which can easily produce a large output of several hundreds mJ or more with a pulse width of 10 ns or less.
The OPO (optical parametric oscillator) and the titanium sapphire (TiS) laser also are used which generate laser beams of various wavelengths with the second harmonic (wavelength of 532 nm) of the Q-switch YAG laser as an excitation source. By use of these lasers, the wavelength dependency of the optical absorption coefficient in the photoacoustic effect can be experimentally determined. An application of this fact to an organism can determine the oxygen saturation degree in the blood. In other words, it is well known that the wavelength dependency of the light absorption amount varies depending on how hemoglobin and oxygen are connected with each other, and by utilizing this phenomenon, a difference between the artery and the vein and a blood vessel newly generated by a tumor can be imaged.
(PTL 1) U.S. Pat. No. 5,713,356